1. Field of the Invention
The present invention relates to an optical add/drop multiplexer. More particularly, the present invention relates to an optical add/drop multiplexer that comprises an optical filter plate and a plane reflector.
2. Description of the Related Art
FIG. 1 is a sketch of a conventional optical add/drop module. As shown in FIG. 1, the module includes a drop multiplexer 10 and an add multiplexer 20. The drop multiplexer 10 includes an input graded-index lens 12, a dropout filter plate 14 and a dropout graded-index lens 16. The add multiplexer 20 includes an output graded-index lens 22, an add filter plate 24, and an add graded-index lens 26. Both the dropout filter plate 14 and the add filter plate 24 are involved in filtering electromagnetic waves of the same wavelength.
A group of incoming optical signals containing signals of various wavelengths is first coupled to the input graded-index lens 12 so that the signals are delivered to the dropout filter plate 14. The filter plate 14 block out all signals except signals at a specified waveband so that the signals at the specified waveband are permitted to pass through the filter 14 and couple with the dropout graded-index lens 16. Signals of the specified waveband or the so-called dropout signals can be read at the other side of the dropout graded-index 16. Meanwhile, blocked signals outside the specified wavelength are reflected back by the dropout filter plate 14.
However, only a portion of signals having the specified wavelength penetrate through the dropout filter plate 14. Hence, a small portion of the signals within the specified waveband will be reflected back by the filter plate 14 together with the rest of the group of signals. Thus, the signal within the specified waveband together with the rest of the group of signals needs to be further deliver to the add filter plate 24. When the group of signals reaches the add filter plate 24, the remaining signals within the specified waveband are able to penetrate through. Consequently, strength of the signals within the specified waveband after reflection from the add filter plate 24 is further reduced to an acceptable low-level. In addition, add-in signals can be added by shining a beam with add-in signals onto the graded-index lens 26. The add-in signals are also within the specified waveband so that the add-in signals can penetrate through the add filter plate 24 and mix with the original group of signals reflected from the add filter plate 24. The add-in signals and the original group of signals emerge from the graded-index lens 22 as a group of output signals. Since the original signals within the specified waveband have been reduced to acceptable low-level, the newly added signals, also within the specified waveband, are unaffected and does not increase the bit error rate.
FIGS. 2a and 2b are patented optical add/drop multiplexers (U.S. Pat. No. 5,712,727). In FIG. 2a, the add/drop multiplexer has two optical filter plates 110 and 120 and four graded-index lenses 112, 114, 122, and 124. Functions of the filter plates 110 and 120 and the graded-index lenses 112, 114, 122, and 124 are similar to the filter plates 14 and 24 and the graded-index lenses 12, 16, 22, and 26 of FIG. 1. When an incoming light beam containing a group of signals at various wavelengths reaches at the first filter plate 110, only the signals within a specified waveband can penetrate through the filter plate 110 and a optical fiber grating 116 to become dropout signals for reading. Meanwhile, signals having wavelengths outside the specified range are reflected by the filter plate 110. In addition, the optical fiber grating 116 further intercept the signals having wavelengths outside the specified range. An optical fiber grating 130 is installed to intercept the reflected light from the filter plate 110 so that residual signals within the specified waveband after first reflection are further reduced. Finally, any remaining signals within the specified waveband are removed on reflection from the second filter plate 120 so that strength of signals within the specified waveband is reduced to an acceptable level. Additional signals can also be added to the reflected signals from the second filter 120 by shining a beam of light containing the add-in signals through filter plate 120. Ultimately, a group of output signals emerges from the second filter 120.
The major disadvantage of this type of optical add/drop multiplexer is that two filter plates 110 and 120 must be used. Furthermore, the group of input optical signals, the group of output optical signals, the add-in signals and the dropout signals must couple with graded-index lenses 112, 114, 122, and 124. Hence, cost of implementation will be high.
The alternative arrangement as shown in FIG. 2b has a filter plate 210, two graded-index lenses 212 and 214, and a 2xc3x971 optical fiber coupler 220. Function of the filter plate 210 is similar to the filter plate 14 as shown in FIG. 1. Signals within a specified waveband in a group of input optical signals are able to penetrate through the filter plate 210 to become dropout signals for reading. The optical fiber grating 216 further filter our undesired signals after the input signals penetrate through the filter plate 210. Other signals contained within the group are reflected by the filter plate 210. An optical fiber grating 230 is installed to intercept the reflected light from the filter plate 210 so that residual signals within the specified waveband are further reduced. Finally, light signals from the optical fiber grating 230 and any additional add-in signals are integrated inside a 2xc3x971 optical fiber coupler 220 so that a group of output optical signals emerges from the coupler 220.
The major disadvantage for this type of optical add/drop multiplexer is the use of a 2xc3x971 optical coupler 220. This is because strength of the group of output optical signals will be reduced by half (representing a 3 dB coupling loss).
The invention provides a type of optical add/drop multiplexer. The multiplexer comprises a first lens 310, a second lens 330 and a third lens 320, a reflector 350, and an optical filtering device 340. The first lens is used for receiving and transmitting a group of optical signals. The group of optical signals contains a spectrum of wavelengths. The optical filtering device includes an optical filter plate and a reflector. The optical filter plate receives the entire group of optical signals, but permits only a specified waveband within the spectrum of wavelengths to penetrate through. Signals from other part of the spectrum are reflected back and forth a number of times between the filter plate and the reflector. On the last reflection from the filter plate, the rest of the group signals with the acceptable low-level signals within the specified waveband emerge as a group of output signals. The second lens picks up the signals within the specified waveband from the filter plate to become dropout signals for reading. The second lens also receives add-in signals having wavelengths within the specified waveband and passes them through the optical filter plate. The third lens integrates the rest of the group signals, the acceptable low-level signals within the specified waveband and the add-in signals from the filter plate, and then transmits the mixed signals out as a group of optical output signals.
This invention also provides a type of optical add/drop multiplexer that includes a glass substrate. An anti-reflection coating is plated over a first planar surface of the substrate while an optical filtering film is coated over a second planar surface of the substrate. The first planar surface and the second planar surface are parallel to each other. The anti-reflection coating receives a group of incoming optical signals and then transmits the signals to the substrate. The group of optical signals includes a spectrum of wavelengths. The optical filtering film picks up the group of transmitted signals from the anti-reflection coating, but permits only signals within a specified waveband of the spectrum to pass through. Signals within the specified waveband penetrate through the optical filtering film as dropout signals for reading. Signals from other part of the spectrum are reflected back and forth inside the substrate between the filtering film and the reflection coating. On the last reflection from the filtering film, the rest of the group signals with the acceptable low-level of specified waveband signals emerge as a group of output signals. The optical filtering film is further capable of receiving add-in signals having wavelength within the specified waveband and then transmitting them to the output coating. The output coating integrates the rest of group signals, the acceptable low-level signals within the specified waveband and the add-in signals from the filter plate, and then transmits the integrated signals out as a group of optical output signals.
Accordingly, the present invention uses just one optical filter plate and a reflector. Multiple optical reflections occur between the filter and the reflector such that intra-band isolation is increased and cost of production is decreased.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.